CPR is a process by which one or more rescuers may provide chest compressions and ventilation to a victim who has suffered an adverse cardiac event—by popular terms, a heart attack. During the first five to eight minutes after CPR efforts begin, chest compressions are considered to be the most important element of CPR because chest compressions help maintain circulation through the body and in the heart itself.
CPR may be performed by a team of one or more rescuers, particularly when the rescuers are professionals, such as emergency medical technicians (EMTs) on an ambulance crew. One rescuer can provide the chest compressions while another can provide and time their ventilations of the victim to match the chest compressions according to the appropriate CPR protocol. When professionals such as EMTs provide the care, ventilation is more likely to be provided via a ventilation bag that a rescuer squeezes rather than by mouth-to-mouth. CPR can be performed in conjunction with shocks to the patient provided by an external defibrillator, such as from an automatic external defibrillator (AED) that is designed to be used by laypeople. Such AEDs often provide audible information to rescuers, such as “push harder” (when the rescuer is not performing chest compressions forcefully enough), “stop CPR,” “stand back” (because a shock is about to be delivered), and so on. In order to determine how chest compressions are being performed, certain defibrillators may obtain information from one or more accelerometers (such as in the CPR D PADZ, CPR STAT PADZ, and ONE STEP pads made by ZOLL MEDICAL of Chelmsford, Mass.) that can be used to compute depths of chest compression (e.g., to determine that the compressions are too shallow to be effective and to thus cause the verbal cue “push header” to be spoken by the defibrillator).